The present invention relates to a noise suppressing device for an FM stereophonic receiver. More particularly, the invention relates to a pulsive noise suppressing device therefor.
FM receivers suffer especially from pulsive noise components such as are caused by ignition plugs or wipers of automobiles. In order to suppress such pulsive noise components, a pulsive noise suppressing circuit is typically provided in which a high-pass filter is provided for detecting or separating pulsive noise components from the output signal from the receiver's FM detector. The noise component separated by the filter is amplified and used to trigger a monostable multivibrator which produces a gate control signal for a predetermined period of time. In response to the gate control signal, a gate circuit is opened to temporarily interrupt the transmission of the detector output signal to the receiver's MPX (multiplex) demodulation circuit. A hold circuit is provided which samples and holds the detector output signal level provided immediately before the opening of the gate for a predetermined period of time after the opening of the gate circuit. The output thus held is substituted for the MPX demodulation input during this period.
In such a pulsive noise suppressing circuit, the detector output signal obtained from the FM detector is employed to suppress the pulsive noise signal. While passing through the many circuits and filters from the RF (radio frequency) stage to the FM detector stage, the phase of the signal is shifted and the width of the noise pulse is increased. Accordingly, it is necessary to prolong the interruption period of the gate circuit. Hence, a significant amount of noise which is attributed to operation of the noise suppressing circuit appears in the audio output signal. This is an undesirable phenomenon. (This phenomenon is illustrated in FIGS. 1A and 1B.)
A stereophonic pilot signal is superimposed on the detector output signal. If the level of the pilot signal is greater than the amplitude of the information signal at the instant of the switch, substantially the level of the pilot signal will be held by the hold circuit and applied to the MPX demodulation circuit for the interruption period of the gate circuit. As a result, a noise signal proportional to the pilot signal level occurs in the audio output. This difficulty may be eliminated by employing a technique in which the pilot signal is extracted from the composite signal and the pilot signal thus extracted is applied to the noise suppressing circuit. However, this technique is still disadvantageous in that a 19 KHz pilot signal processing circuit is necessary in order to accurately transmit the pilot signal to the MPX demodulation circuit.
Accordingly, an object of the invention is to provide a pulsive noise suppressing device for an FM stereophonic receiver in which noise signals can be suppressed more completely but in which a 19 KHz pilot signal processing circuit is not required.